14 But in this same matter, touching Christian faith, there are other duties whose exact and religious observance, necessary at all times in the interests of eternal salvation, become more especially so in these our days. Amid such reckless and widespread folly of opinion, it is, as We have said, the office of the Church to undertake the defense of truth and uproot errors from the mind, and this charge has to be at all times sacredly observed by her, seeing that the honor of God and the salvation of men are confided to her keeping. But, when necessity compels, not those only who are invested with power of rule are bound to safeguard the integrity of faith, but, as St. Thomas maintains: "Each one is under obligation to show forth his faith, either to instruct and encourage others of the faithful, or to repel the attacks of unbelievers."(12) To recoil before an enemy, or to keep silence when from all sides such clamors are raised against truth, is the part of a man either devoid of character or who entertains doubt as to the truth of what he professes to believe. In both cases such mode of behaving is base and is insulting to God, and both are incompatible with the salvation of mankind. This kind of conduct is profitable only to the enemies of the faith, for nothing emboldens the wicked so greatly as the lack of courage on the part of the good. Moreover, want of vigor on the part of Christians is so much the more blameworthy, as not seldom little would be needed on their part to bring to naught false charges and refute erroneous opinions, and by always exerting themselves more strenuously they might reckon upon being successful. After all, no one can be prevented from putting forth that strength of soul which is the characteristic of true Christians, and very frequently by such display of courage our enemies lose heart and their designs are thwarted. Christians are, moreover, born for combat, whereof the greater the vehemence, the more assured, God aiding, the triumph: "Have confidence; I have overcome the world."(13) Nor is there any ground for alleging that Jesus Christ, the Guardian and Champion of the Church, needs not in any manner the help of men. Power certainly is not wanting to Him, but in His loving kindness He would assign to us a share in obtaining and applying the fruits of salvation procured through His grace.
Source: Sapientiae Christianae (Vatican.va)